muffin help

Hi, I really seek your help on this banana walnut muffin. I am not sure whether is this not cook yet. Any advice on it. here are some photos on it and the recipe and the method. please help! thanks!! ( it was sticky not crumbly)

recipe

1) 150g of plain flour

2) 7.5 ml of baking powder

3) 55g butter, softened

4) 150g castor sugar

5)1egg

6) 5ml of vanilla extract

7)3bananas, mashes

8)75ml of milk

9) 55g of walnuts

Method

1.preheat oven 190 degrees

2.sift flour and baking powder

3. beat butter and sugar till light and fluffy. ( when i cream butter and sugar, it just come out creamy. is it right? i mix it for 3 mins already, but did not come out fluffy as usual.)

4.add egg, vanilla and beat till smooth. add bananas

5. with the mixer on low speed, beat in the flour mixture, alternating it with the milk. add walnuts.

Butter should be at room temperature before starting recipe. When beating butter and sugar, the mixture should be creamy and fluffy...this takes time. Add the flour and baking powder all at once and stir just to incorporate. Do not over mix. You can even have some undissolved flour peaking out from the batter. Over mixing causes tough muffins. Bake until just done. Over baking can cause texture problems as well.